You're a web developer and you cannot live without Firebug
But what if you have problems specific to IE, Opera or Safari? Since Firebug is written as an Add-On for Firefox you cannot use it in other browsers. Actually that's no true :) In such a case you can use Firebug Lite
There are several ways to use Firebug Lite, just go to the main website to check them out. The best one (in my opinion) is to use the Firebug Lite bookmarklet, which you simply drop to your bookmarks area of the browser (or save as favorite in IE). Just drag&drop it...